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Method For Desulphurization Of Gases - Patent 6656249

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The present invention relates to a method of removing sulphur compounds from a gas stream. In this method, the sulphur compounds are first converted into hydrogen suilphide, and the hydrogen sulphide is then, in an aqueous solution, convertedinto elemental sulphur by biological oxidation.BACKGROUND OF THE INVENTIONAs far as the oil and gas(-processing) industry is concerned, sulphur compounds are among the most important pollutants which can occur in off-gases. It is a pollutant which occurs at high concentrations, and legislation relating to thesecompounds is very strict.Consequently, many processes are known for removing sulphur compounds from gasses. One of the most important processes comprises the catalytic conversion of the sulphur compounds to elemental sulphur. A major advantage of this process is thatthe elemental sulphur is a product of intrinsic economic value.The most important process for converting sulphur compounds, especially hydrogen sulphide, into elemental sulphur is the so-called Claus process. Using this process, a total sulphur removal of about 95% can be achieved. The residual quantity ofsulphur is contained in the so-called "Claus off-gas" (sometimes referred to as "Claus tail gas") in the form of sulphur compounds such as COS, CS.sub.2, SO.sub.2 and SO.sub.3, but also small amounts of gaseous elemental sulphur (S.sub.x) and mercaptans(RSH).In other processes too, sulphur-containing gas streams can be formed, such as synthesis gas or fuel gas, which still contain the abovementioned undesirable sulphur compounds. These sulphur compounds are often converted into hydrogen sulphide bymeans of catalytic hydrogenation or catalytic hydrolysis.An example of the catalytic hydrogenation of these compounds to produce hydrogen sulphide is the so-called SCOT process (Shell Claus Off-gas Treatment). This process converts the sulphur compounds present in the off-gas of the Claus process. Usually, the hydrogen sulphide thus formed, having been select

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United States Patent: 6656249


































 
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	United States Patent 
	6,656,249



 Buisman
,   et al.

 
December 2, 2003




 Method for desulphurization of gases



Abstract

A method of removing hydrogen sulphide from a gas stream wherein the
     hydrogen sulphide is scrubbed from the gas phase by an aqueous solution,
     the hydrogen sulphide in the aqueous solution is biologically oxidized in
     a bioreactor to produce elemental sulphur, and the elemental sulphur is
     separated from the aqueous solution, characterized in that the gas stream
     to be treated is cooled to such a degree that at least sufficient water
     vapour condenses from the gas stream to compensate for the discharge
     stream for the purpose of removing salts. This means that no water need be
     supplied to the bioreactor. This method is suitable, in particular, for
     gas streams which contain hydrogen sulphide, the hydrogen sulphide having
     been obtained by catalytic conversion of sulphur compounds.


 
Inventors: 
 Buisman; Cees Jan Nico (Harich, NL), Janssen; Albert Joseph Hendrik (Sneek, NL), Van Bodegraven; Robert Jan (Harich, NL) 
 Assignee:


Paques Bio Systems B.V.
 (Balk, 
NL)





Appl. No.:
                    
 09/936,204
  
Filed:
                      
  September 10, 2001
  
PCT Filed:
  
    March 08, 2000

  
PCT No.:
  
    PCT/NL00/00155

      
PCT Pub. No.: 
      
      
      WO00/53290
 
      
     
PCT Pub. Date: 
                         
     
     September 14, 2000
     


Foreign Application Priority Data   
 

Mar 08, 1999
[NL]
1011490



 



  
Current U.S. Class:
  95/195  ; 210/622; 95/205; 95/235; 96/234
  
Current International Class: 
  C01B 17/05&nbsp(20060101); C01B 17/00&nbsp(20060101); B01D 53/48&nbsp(20060101); B01D 53/52&nbsp(20060101); B01D 53/84&nbsp(20060101); B01D 053/14&nbsp()
  
Field of Search: 
  
  







 210/622,620 95/156,195,205,202,235 96/234
  

References Cited  [Referenced By]
U.S. Patent Documents
 
 
 
5474682
December 1995
Buisman

5747331
May 1998
Hartikainen et al.

5958238
September 1999
Langerwerf

6156205
December 2000
Buisman et al.



 Foreign Patent Documents
 
 
 
WO 97/43033
Nov., 1997
WO

WO 98/57731
Dec., 1998
WO



   Primary Examiner:  Smith; Duane S.


  Attorney, Agent or Firm: Young & Thompson



Claims  

What is claimed is:

1.  A method of removing hydrogen sulfide from a gas stream, comprising scrubbing a gas stream containing hydrogen sulfide with an aqueous solution, biologically oxidizing the
hydrogen sulfide in the aqueous solution in a bioreactor to produce elemental sulfur, separating the elemental sulfur thus produced from the aqueous solution, and cooling the gas stream to such a degree that at least sufficient water vapor condenses from
said gas stream to add water to a discharge stream from said bioreactor for the purpose of removing salts.


2.  A method as claimed in claim 1, comprising performing said cooling step of said gas stream in a quench column, and passing at least a portion of the condensate water obtained in the quench column to the bioreactor.


3.  A method as claimed in claim 1, and performing said cooling step of said gas stream in a quench column to a temperature higher than the temperature of the bioreactor.


4.  A method as claimed in claim 1, and carrying out said scrubbing under such process conditions that an emerging scrub liquor in which hydrogen sulfite has been absorbed is suitable for biological oxidation.


5.  A method according to claim 1, and keeping the conductivity of the aqueous solution constant by said condensation of water vapor.


6.  A method as claimed in claim 1, and obtaining said hydrogen sulfide at least in part by catalytic conversion of sulfur compounds.


7.  A method as claimed in claim 6, and performing said catalytic conversion of sulfur compounds by catalytic hydrogenation.


8.  A method as claimed in claim 7, wherein said sulfur compounds to be removed are selected from the group consisting of sulfur dioxide, sulfur trioxide, carbonyl sulfide, carbon disulfide, mercaptans and sulfur vapor.


9.  A method as claimed in claim 6, wherein said conversion of sulfur compounds is performed by catalytic hydrolysis.


10.  A method according to claim 9, wherein said sulfur compounds to be removed are selected from the group consisting of carbonyl sulfide, carbon disulfide and mercaptans.


11.  A method according to claim 1, and recirculating to the bioreactor aqueous solution from which elemental sulfur has been separated.


12.  A method according to claim 1, and keeping the temperature of the bioreactor to a value that does not exceed 65.degree.  C.  Description  

FIELD OF THE INVENTION


The present invention relates to a method of removing sulphur compounds from a gas stream.  In this method, the sulphur compounds are first converted into hydrogen suilphide, and the hydrogen sulphide is then, in an aqueous solution, converted
into elemental sulphur by biological oxidation.


BACKGROUND OF THE INVENTION


As far as the oil and gas(-processing) industry is concerned, sulphur compounds are among the most important pollutants which can occur in off-gases.  It is a pollutant which occurs at high concentrations, and legislation relating to these
compounds is very strict.


Consequently, many processes are known for removing sulphur compounds from gasses.  One of the most important processes comprises the catalytic conversion of the sulphur compounds to elemental sulphur.  A major advantage of this process is that
the elemental sulphur is a product of intrinsic economic value.


The most important process for converting sulphur compounds, especially hydrogen sulphide, into elemental sulphur is the so-called Claus process.  Using this process, a total sulphur removal of about 95% can be achieved.  The residual quantity of
sulphur is contained in the so-called "Claus off-gas" (sometimes referred to as "Claus tail gas") in the form of sulphur compounds such as COS, CS.sub.2, SO.sub.2 and SO.sub.3, but also small amounts of gaseous elemental sulphur (S.sub.x) and mercaptans
(RSH).


In other processes too, sulphur-containing gas streams can be formed, such as synthesis gas or fuel gas, which still contain the abovementioned undesirable sulphur compounds.  These sulphur compounds are often converted into hydrogen sulphide by
means of catalytic hydrogenation or catalytic hydrolysis.


An example of the catalytic hydrogenation of these compounds to produce hydrogen sulphide is the so-called SCOT process (Shell Claus Off-gas Treatment).  This process converts the sulphur compounds present in the off-gas of the Claus process. 
Usually, the hydrogen sulphide thus formed, having been selectively removed by means of an amine-containing solution, is recycled to the Claus reactor to increase the efficiency of the latter.


Even the off-gas of said SCOT process may still contain traces of sulphur components, generally traces of COS and CS.sub.2.  To prevent emission of these compounds, this process will generally comprise an afterbuming step, in which these
compounds are converted into SO.sub.2.


Alternatively, the undesirable sulphur compounds can be converted to hydrogen sulphide by catalytic hydrolysis.  Such a process is used, in particular, in gas streams which contain carbonyl sulphide (COS).  Characteristic gas phase hydrolysis
catalysts are based on copper sulphide, chromium oxide, chromium oxide/aluminium oxide, and platinum.


Another example of a process in which sulphur compounds present in the Claus off-gas are converted to hydrogen sulphide is the so-called Beavon process.  This process is used to remove sulphur compounds from off-gas of the Claus process by means
of hydrolysis and hydrogenation over a cobalt molybdate catalyst, which results in the conversion of carbonyl sulphide, carbon disulphide and other sulphur compounds into hydrogen sulphide.


OBJECT OF THE INVENTION


It is an object of the present invention to provide a method of converting hydrogen sulphide into elemental sulphur, said method making use of aerobic bacteria.  Consequently, the resultant H.sub.2 S from the catalytic reduction step will not be
recycled to the Claus reactor.  This application is of interest, in particular, if a Claus installation of this type is unavailable, as is the case, for example, with so-called stand-alone SCOT units.


Another object of the present invention is to carry out this conversion in such a way as to thereby achieve high efficiency.


SUMMARY OF THE INVENTION


A method of removing hydrogen sulphide from a gas stream has now been found wherein the hydrogen sulphide is scrubbed from the gas phase by means of an aqueous solution, the hydrogen sulphide in the aqueous solution is biologically oxidized in a
bioreactor to produce elemental sulphur, and the elemental sulphur is separated from the aqueous solution, characterized in that the gas stream to be treated is cooled to such a degree that at least sufficient water vapour condenses from said gas stream
to compensate for the discharge stream for the purpose of removing salts.  This means that no water need be supplied to the bioreactor, and it may even be possible to produce good-quality water.


The method according to the present invention further provides the following advantages: Using this method, it is possible for elemental sulphur to be obtained with a high yield from gas streams which contain hydrogen sulphide.  Any HCN present
in the gas stream reacts with elemental sulphur to form thiocyanate (SCN.sup.-) which is biodegraded.  Even traces of other sulphur compounds still present in the gas stream are converted.  The process has low energy consumption.  No expensive chemicals
are required.  The process can be operated in a simple manner. 

BRIEF DESCRIPTION OF THE DRAWINGS


To clarify the present method, the following figures are appended:


FIG. 1, which depicts the decay rate of the biologically produced sulphur as a function of the pH and temperature.


FIG. 2 which depicts an embodiment of the present invention. 

DETAILED DESCRIPTION OF THE INVENTION


It is preferable for the hydrogen sulphide to have been obtained by catalytic conversion of sulphur compounds.


The sulphur compounds are converted, preferably by catalytic hydrogenation, to hydrogen sulphide.  This conversion is suitable, in particular, when the sulphur compounds comprise sulphur dioxide (SO.sub.2), sulphur trioxide (SO.sub.3), carbonyl
sulphide (COS), carbon disulphide (CS.sub.2) and sulphur vapour (S.sub.x).  Preferably, the sulphur compounds are converted to hydrogen sulphide by means of the hydrogenation step in the above-described SCOT process.


Alternatively, the sulphur compounds can be converted to hydrogen sulphide by catalytic hydrolysis.  Catalytic hydrolysis is suitable, in particular, if the gas stream comprises carbonyl sulphide (COS) and possibly carbon disulphide (CS.sub.2)
and mercaptans (RSH).


Biological oxidation of hydrogen sulphide to elemental sulphur is known.  Such methods are described, for example, in WO 96/30110 and WO 92/10270.


A Claus unit designed for 100 T/D of sulphur generally produces about 13,000 M.sup.3 (s.t.p.) of off-gas.  After conversion in the catalytic reduction reactor the off-gas contains 2-8 T/D of sulphur, depending on the efficiency of the upstream
Claus unit.  About a third of the volume of the gas is water.  The temperature of the gas is 200-340.degree.  C., depending on the catalyst and the conversion requirements.  A typical value of the dew point of the gas is between 65 and 75.degree.  C.
This gas must be cooled to a temperature at which it will no longer adversely affect the biomass bioreactor.  This cooling is preferably carried out in a quench column, the gas stream being brought into contact with a circulating water stream which is
cooled by an external cooler.  Upstream of the quench column there may be a heat recovery boiler, if good use can be made of the steam.  The temperature of the quench water system is between 25 and 65.degree.  C. and is preferably low enough for a
sufficient amount of water to be condensed to eliminate the need for make-up water.


The quench column serves to cool the gas, thus preventing excessive uptake of undesired components such as sulphur dioxide and ammonia.  These components can be present in the gas owing to variations in the upstream equipment and can have a
negative effect on the biosystem.  The quench column can also be used for the recovery of water.  The water produced in the quench column, after a simple step involving steam stripping, is of very good quality and can be used as feed water for a boiler
or can be stored as clean water in a reservoir.


Part of the H.sub.2 S absorbed in the biosystem is oxidized to sulphatc.  This is formed as a result of the undesirable oxidation to the highest state of sulphur according to:


The sulphate production is from about 3 to 10% of the total sulphide load.  To prevent acidification of the medium in the reactor, the sulphate produced needs to be neutralized, for example with sodium hydroxide or sodium carbonate.  The sodium
sulphate formed needs to be discharged from the system.  The essence of the patent application is based on such cooling of the sour gas to below its dew point that enough condensate water is formed to compensate for the discharge stream.


The make-up water required to compensate for this discharge can be supplied from the quench column.  It is also possible for the temperature of the quench column to be set to a higher value than that of the bioreactor, so that enough water will
condense in the scrubbing column.


As described above, the hydrogen sulphide is scrubbed from the gas phase by means of an aqueous solution.  This step can be carried out in a gas scrubber in which intensive contact is effected between the gas stream and the scrub liquor.


If required, the scrub liquor can be buffered to a pH of between 6.0 and 10.0.


The buffering compounds must be tolerated by the bacteria present in the oxidation reactor.  Preferred buffering compounds are carbonates, bicarbonates, phosphates and mixtures thereof, in particular sodium carbonate and/or sodium bicarbonate. 
The concentration of the buffering compounds is generally set to a value of between 20 and 2000 meq/l. If sodium carbonate is employed as the buffering compound, its concentration is preferably set to from about 15 to 25 g/l. Where the present
description refers to concentrations of bicarbonate and carbonate, these are expressed, respectively, as the concentration by weight of the ions HCO.sub.3 .sup.31 and CO.sub.-.  The ratio of HCO.sub.3.sup.- to CO.sub.3 depends on the pH of the solution,
which in turn is determined by the partial pressure of CO.sub.2 and H.sub.2 S of the gas stream to be treated.


The addition of buffering compounds can take place after the scrub liquor has left the gas scrubber, but also before it is passed into the gas scrubber.


It is necessary for the moist gas to be cooled so as to attain the desired temperature in the bioreactor.  The desired equilibrium temperature of the suspension in the bioreactor depends on (1) the temperature at which the microorganisms are
still active, and (2) the chemical stability of the sulphur formed.  Laboratory studies have established that Thiobacilli are capable, up to a temperature of 70.degree.  C., of oxidizing the sulphide.  At these high temperatures, however, the sulphur
formed will hydrolyse to a considerable extent, in accordance with the following reaction equation:


In addition, it is also possible for sulphide (HS.sup.-) and sulphate (SO.sub.4.sup.2-) to be formed, in accordance with:


 4S.sup.o +5OH.sup.-.fwdarw.3HS.sup.- +SO.sub.4.sup.2- +H.sub.2 O. (2)


Laboratory experiments, carried out with sulphur formed biologically, produced by a THIOPAQ reactor, have shown that chiefly sulphide (HS.sup.-) and thiosulphate (S.sub.2 O.sub.3.sup.2-) are formed, and the assumption is therefore that reaction
(1) is the main one to take place.


FIG. 1 depicts the rate of decay of the biologically produced sulphur as a function of the pH and the temperature.


As the chemical stability of the sulphur produced decreases with increasing pH and temperature, the temperature of the suspension in the bioreactor must not exceed 65.degree.  C.


As bacteria which, if the scrub liquor is treated in the presence of oxygen, oxidize sulphide to elemental sulphur (here referred to as sulphur-oxidizing bacteria), the autotrophic aerobic cultures known for this purpose are potentially suitable,
such as those of the genera thiobacillus and thiomicrospira.


It is advantageous for the specific conductivity of the aqueous solution in which the hydrogen sulphide is absorbed to be constant.  The specific conductivity is a measure for the total amount of dissolved salts.  This chiefly relates to sodium
(bi)carbonate and sodium sulphate.  The specific conductivity should be controlled within a range from 10 to 100 mS/cm, preferably between 40 and 70 mS/cm.


The quantity of oxygen added to the scrub liquor is controlled so as to ensure that the oxidation of the absorbed sulphide mainly gives rise to elemental sulphur.  Such a method of controlled oxidation of sulphur-containing waste water is
described in the Dutch patent application 8801009.


The formation of sulphur in the oxidation reactor leads to a sulphur suspension, which is drawn off.  The sulphur from this suspension is separated from the aqueous solution by filtration, centrifuging, flocculation, settling, etc. After
separation, the sulphur can be further processed by drying and possible purification, and be re-used.  The remaining liquor can be re-used as scrub liquor.


It proves beneficial for not all the sulphur to be drawn off and the drawing off to be carried out discontinuously or in part, thereby producing a scrub liquor which still contains sulphur.  The sulphur concentration in the scrub liquor is
generally kept between 0.1 and 50, preferably between 1 and 50, more preferably between 5 and 50 g/l (from 1 to 5 wt %).  In particular, the percentage of sulphur separation is controlled in such a way that as much scrub liquor as possible is re-used. 
The liquor recovered when the drawn-off sulphur is processed can, if appropriate, be added to the scrub liquor.


addition to hydrogen sulphide, the gas may also contain hydrogen cyanide gas (HCN).  Especially in the event of HCN being present as a component in the gas, elemental sulphur in the scrub liquor is beneficial.  The cyanide, which is toxic to most
bacteria, is thereby converted into the much less toxic thiocyanate which is subseqaently broken down biologically and/or chemically.  Ultimately, HCN is converted into carbon dioxide and nitrate.


The sulphide concentration, expressed as sulphur, in the scrub liquor used, having a pH of about 8.5, will usually be about 15-3000 mg/l when gases of roughly atmospheric pressure are cleaned.


The ratio of the amounts of scrub liquor to gas is determined, on the one hand, by the absorption capacity of the scrub liquor with respect to H.sub.2 S and, on the other hand, by hydrodynamic characteristics of the gas scrubber.


The gas scrubbers to be used according to the invention can be of a customary type, as long as an effective contact is achieved, in the gas scrubbers, between the gas stream and the scrub liquor.


Preferably, use is made, for the method according to the invention, especially for the aerobic reactor(s), of reactors of the vertical circulation type as described, for example, in the International patent application 94/29227, wherein the gas
to be used (in the aerobic reactor this is usually air) is able to provide the vertical circulation.


FIG. 2 depicts a possible embodiment of the method according to the present invention, wherein the hot gas is cooled in a quench 1 combined with a cooler 2, a second cooler 3 being positioned, if required, between the gas scrubber (=absorber) 4
and the THIOPAQ bioreactor 5.  The water which condenses in the quench 1 is passed, in its entirety or in part, to the bioreactor 5.  Any excess water can be discharged via 18 and, after stripping of H.sub.2 S, be used elsewhere on site as process water.


The gas stream 7 to be treated is cooled in quench 1 to below the dewpoint (65-75.degree.  C.) by cooling water which is recirculated via the quench being cooled via 10 by means of cooler 2.  Via 8, the condensate is passed to the bioreactor 5 to
serve as make-up water so as to eliminate the sulphates formed.  Any excess water is discharged via 18.  Via 9, the cooled gas is directed to the gas scrubber 4 where the gas is scrubbed with stream 15 which, via 17, is admixed with a small amount of
sodium hydroxide or sodium carbonate.  In the gas scrubber, the H.sub.2 S is efficiently scavenged.  Emerging from the gas scrubber is a gas stream 11 and the H.sub.2 S-containing scrub water which is passed from the gas scrubber via 12, together with
the condensate, to the bioreactor 5.  If necessary, this stream is further cooled with the aid of cooler 3.


In the bioreactor, the H.sub.2 S is oxidized to elemental sulphur and, to a small extent, to sulphate.  For the purpose of the oxidation, air is introduced into the bioreactor via 20.  The exhaust air is discharged via 21.  The sulphate formed is
eliminated via 14, together with the condensate required therefore.  A substream from the bioreactor is directed, via 13, to a sulphur recovery unit 6 where the sulphur is separated via 19.  From the sulphur recovery unit 6, part of the recovered sulphur
is recycled to the bioreactor 5 via line 16.


According to this embodiment, the quench 1 and the gas scrubber 4 are positioned above one another in the same column, the water circulation remaining separate within the two sections.


The water content in the hot gas on average is about 33 mol%, which is amply sufficient.  The amount of water which condenses can be controlled by setting the temperature in the coolers.  The cooler the temperature set, the more water will
condense.


EXAMPLES


In the table below, two gas streams are used as an example.  These gas streams have been catalytically treated before H.sub.2 S is removed and converted biologically into elemental sulphur.


 TABLE I  Gas streams as an example of treatment in a  catalytic/biological desulphurization process  1 2  Type of gas Claus off-gas Syngas  Composition  (dry-gas analysis)  (in vol %)  H.sub.2 S 0.45 1.9  SO.sub.2 0.22 --  COS 75 ppm 0.1 
CS.sub.2 60 ppm 300 ppm  CH.sub.3 SH -- 50 ppm  S.sub.x 600 ppm --  CO 0.22 29  CO.sub.2 3 26  H.sub.2 2.2 40  N.sub.2 94 0.9  CH.sub.4 -- 2


Example 1


Claus Off-gas


After catalytic conversion, Claus off-gas (20,000 m.sup.3 (s.t.p.)/h) has a temperature of 200.degree.  C. About 4 tonnes of sulphur are removed as H.sub.2 S each day.  The hot gas contains 33 vol% of water vapour.  In the Thiopaq reactor, 25 kg
of SO.sub.4 /h is formed, based on 5% oxidation to the highest state.  With a standard sulphate concentration in the bioreactor of 18 kg of SO.sub.4 /m.sup.3, the discharge stream will be 1.4 m.sup.3 /h. Cooling the gas to 63.degree.  C. will produce the
same amount of condensate water.  Cooling is effected as follows: by means of a quench, the gas is cooled to its dew point of about 70.degree.  C. The biological system is operated at 50.degree.  C., thus causing the temperature of the gas in the
absorber to drop to 63.degree.  C. The scrub water used will warm up in the process, which means that a heat exchanger will be required.


Example 2


Syngas


After catalytic conversion, syngas has a temperature of 160.degree.  C. and a flow rate of 6000 m.sup.3 (s.t.p.)/h. About 4 tonnes of S are removed as H.sub.2 S each day.  The gas contains 30 vol% of water vapour.


Maintaining the conductivity at the desired level requires a discharge stream of 1.4 m.sup.3 /h. Because the amount of gas is much lower than in the previous example, cooling of the gas takes place directly in the absorber.  Cooling the gas to
56.degree.  C. causes condensation of enough water vapour to obtain the desired discharge stream.  Cooling is achieved by the biological system being operated at 48.degree.  C. The heated scrub water is cooled by means of a heat exchanger.


Example 3


An amount of gas (31,574 m.sup.3 (s.t.p.)/h) from a hydrogenation reactor has the following composition: 1.24 vol% of H.sub.2 S 2.02 vol% of H.sub.2 12.64 vol% of CO.sub.2 56.65 vol% of N.sub.2 0.67 vol% of Ar 26.80 vol% of H.sub.2 O


The temperature is 317.degree.  C. and the pressure is 1.10 bar (abs).


If this gas is cooled to 32.degree.  C., the water content drops to 4.55 vol %. This means that 5.89 m.sup.3 /h of water will condense.  If 3.5% of the captured H.sub.2 S oxidizes to a sulphate, 57 kg/h of sulphate are formed.  For a sulphate
content of 25 kg/M.sup.3, the discharge stream will be 2.3 m.sup.3 /h. The net production of water which can be used for other purposes is 3.59 m.sup.3 /h.


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